The palaestra was the training ground for wrestlers, boxers, and the jumpers who competed in the pentathlon. Built in the 3rd century BC, it consists of a large square courtyard surrounded by a Doric colonnade with changing rooms, bathing facilities, and halls for training off to the sides. Several column stumps remain, though the site conveys its original layout clearly enough that you can picture athletes oiling their skin and coating themselves in sand before a bout.
Unlike the temples nearby, the palaestra was a working facility — a gymnasium in the literal Greek sense, a place of naked training. Athletes were required to arrive at Olympia a month before the Games to train under the supervision of official coaches. The palaestra and the adjacent gymnasium (now largely lost) formed the complex where this mandatory preparation took place.